Two main types of gearbox of the above mentioned type are known for example from U.S. Pat. Nos. 5,150,628 and 5,388,472. Both these gearbox types are so-called "power shift" gearboxes in which the torque is transmitted alternately via both the intermediate shafts to the output shaft, which means that whilst the torque is being transmitted by one of the intermediate shafts the next gear is preselected on the other intermediate shaft, whereby gear changing itself occurs in that the first intermediate shaft is disengaged and the other is engaged. This can be achieved either by driving the input shaft of each of the intermediate shafts and alternately engaging the input shafts via a pair of clutches outside the gearbox as shown in the aforementioned U.S. Pat. No. 5,388,472, or by driving both intermediate shafts from a common input shaft via friction clutches arranged within the gearbox housing as shown in the aforementioned U.S. Pat. No. 5,150,628.
A characteristic of these known gearboxes is that the torque in a gear stage, namely the lowest gear, is transmitted via both intermediate shafts by means of a common gear wheel. In this way an additional reduction stage is achieved compared with the torque transmission, via only one intermediate shaft at a time, which allows the possibility of higher gearing in the lowest gear than can otherwise be obtained. Alternatively, for a given gearing, larger gear wheels can be used than with torque transmission via an intermediate shaft. The construction gives an extra gear stage without the gearbox needing to be complemented with additional cooperating gear wheels. The result is a very short and compact gearbox with a large number of gears.